The present invention relates to a reproducing power adjusting method, an optical information recording/reproducing apparatus and an information recording medium, and more particularly to a reproducing power adjusting method using super-resolution techniques of reproducing pits of a size smaller than an optical resolution with laser radiation heat, an optical information recording/reproducing apparatus having a unit for adjusting a reproducing power, and an information recording medium storing information to be used for performing reproducing power adjustment.
Optical discs are widely used as information recording media. Signals are recorded in an optical disc or recorded signals are reproduced by condensing a laser light beam with an objective lens and radiating the condensed laser beam to an information recording layer of an optical disc. A size of a condensed spot, on an optical disk is represented by λ/4 NA where λ is a wavelength of a laser beam and NA is an numerical aperture of an objective lens. If recurrent patterns of data pits and spaces having the same length are reproduced by using the λ/4 NA spot, the minimum size of a data pit being able to obtain a finite reproduced signal amplitude is λ/4 NA. A size smaller than λ/4 NA is called a size smaller than an optical resolution. According to conventional optical disc technologies represented by CDs, DVDs, HD-DVDs, Blu-ray Discs (BDs), the minimum size of a data pit is set to an optical resolution or larger. High density recording of a conventional optical disc has been attained by shorter wavelength of a laser beam. For example, for CD having a record capacity of 0.65 GB, a wavelength of a laser beam is 780 nm, and for BD having a record capacity of 25 GB, a wavelength of a laser beam is 405 nm. In addition, high density recording is realized by increasing a numerical aperture of an objective lens from 0.5 to 0.85 to reduce the size of a condensed spot. In order to increase the capacity per one optical disc, multilayer providing two information layers realizes large capacities of 8.5 GB, 30 GB and 50 GB of DVD, HD-DVD, and BD respectively. In reproducing a multilayer disc, laser radiation is focused to each layer.
As one method of realizing high density recording other than that described above, super-resolution techniques have been proposed. According to super-resolution techniques, it becomes possible to reproduce pits of a size smaller than an optical resolution, by providing some mechanism to an optical disc medium. As a laser beam is radiated to an optical disc having a phase transition film formed on a ROM type substrate, thermal distribution in an optical spot melts only a portion of phase transition material in the spot so that optical characteristics change: such as a refractive index and a reflection ratio of only a portion of the phase transition material. As a reproducing beam is radiated to a region including an area having different optical characteristics, a state of a reflected beam changes greatly more than the region not including an area having different optical characteristics. It is therefore possible to reproduce pits of a size smaller than an optical spot, i.e., pits of a size smaller than an optical resolution. Super-resolution techniques are therefore techniques of reproducing fine pits with reproducing laser radiation heat. Substance to be used for realizing super-resolution and changing optical characteristics with temperature is called super-resolution substance, and a super-resolution substance film formed on an optical disc is called a super-resolution film. In super-resolution reproducing, an area having different optical characteristics of a medium in an optical spot radiation area is called a super-resolution spot. Reproducing a pit or record mark of a size of an optical resolution or larger is called normal resolution reproducing. Conventional optical disc techniques of reproducing products such as CDs, DVDs, HD-DVDs, BDs are all normal resolution reproducing.
It is known that in super-resolution reproducing by super-resolution techniques, the quality of a reproduced signal changes with a reproducing power. This is because a reproducing power changes the state of a super-resolution spot, e.g., the size, shape and the like. In super-resolution reproducing, there exists therefore an optimum reproducing power which optimizes the state of a super-resolution spot and realizes optimum reproducing. The optimum reproducing power changes with a type and sensitivity of super-resolution substance of a medium, a reproducing linear velocity, an environment temperature and the like during reproducing. The reason for this is as follows. If any of the above-described factors is different, an optimum state of a super-resolution spot is also different so that a radiation amount of a reproducing laser beam necessary for obtaining an optimum super-resolution spot becomes different. The factors changing an optimum reproducing power in super-resolution reproducing, including the above-described type and sensitivity of super-resolution substance of a medium, reproducing linear velocity, environment temperature and the like during reproducing, are collectively called hereinafter reproducing conditions.
As the optimum reproducing condition is determined, super-resolution reproducing is performed by using the optimum reproducing power under the determined optimum reproducing condition to realize optimum reproducing. However, in actual optical disc reproducing, the reproducing condition may be changed during a reproducing operation. For example, a sensitivity of a medium may change with a radius of an optical disc, resulting from a thickness difference between the inner and outer circumference sides of a super-resolution film formed on a substrate or from other reasons. In such a case, reproducing power adjustment is required to change a reproducing power to an optimum reproducing power during a reproducing operation.
JP-A-2002-92994 describes that in super-resolution reproducing for magneto optical recording, test data is reproduced at different reproducing powers, reproducing powers allowing each error rate to become about an error rate capable of being corrected by an error correcting unit, and resolutions corresponding to the reproducing powers (resolution being an amplitude ratio of the shortest signal to the longest signal), are stored in a memory, a resolution is calculated from the reproduced signals during super-resolution reproducing, and compared with a resolution (hereinafter called a target resolution) acquired during test reading, and if the resolution varies from the target resolution, the reproducing power is changed to make the resolution become coincident with the target resolution. A resolution during the reproducing operation becomes therefore always constant, and the reproducing power is always the optimum reproducing power, realizing optimum reproducing.
JP-A-2001-160232 discloses a reproducing power adjusting method using as an observation index for observing a change in the reproducing condition of super-resolution reproducing, a carrier level normalized by a reproducing power. In the invention disclosed in JP-A-2001-160232, the reproducing power is adjusted in such a manner that a signal level (carrier level) normalized by a reproducing power becomes coincident with the target value. Since the signal level is a level capable of being acquired from a reproduced signal during the reproducing operation, the signal level is able to be used also as the observation index for a super-resolution reproducing condition.
As described above, it is possible to realize optimum reproducing of super-resolution reproducing by performing reproducing power adjustment for changing a reproducing power always to the optimum reproducing power in accordance with the reproducing condition.